In the field of photodetection, there is an increasing demand for a technology capable of performing detection of near infrared light at low cost. The detection of near infrared light is expected to be applied to, for example, automatic driving technology for automobiles. An example of a photodetection element of the related art capable of detecting near infrared light includes a photodetection element using a compound semiconductor including an element of Group II to Group VI, but a process of fabricating a photodetection element using a compound semiconductor is complicated and expensive. Further, in a general PN junction type photodetection element, it is difficult to detect light having a wavelength longer than a band gap of silicon (equal to or greater than 1100 nm), and even in a photodetection element using black silicon, a detection limit at a longer wavelength is about 1200 nm.
On the other hand, in a Schottky type photodetection element using a Schottky junction, free electrons are excited due to light absorption inside a metal film, and the generated excited electrons (hot carriers) flow as a photocurrent to a semiconductor side beyond the Schottky barrier. Since a detection limit on the long wavelength side is determined by a height of the Schottky barrier, light with a wavelength longer than a band gap of silicon can be detected. Examples of such a Schottky type photodetection element include photodetection elements described in Japanese Patent No. 4789752, Japanese Unexamined Patent Publication No. 2000-164918, Japanese Unexamined Patent Publication No. 2011-77274, and Japanese Patent No. 2659184. Such a photodetection element of the related art is configured as a back-surface incidence type photodetection element in which a metal film that forms a Schottky junction with the semiconductor layer is provided on the surface of the semiconductor layer, and a back surface of the semiconductor layer is a light incidence surface. A periodic concave/convex structure is provided on the surface of the semiconductor layer, and a surface area of the metal film with respect to incident light is extended to achieve improved sensitivity.